Mating behaviour and the reproductive ecology of the big-handed crab, Heterozius rotundifrons A. Milne Edwards, 1867

Abstract

The mating behaviour and reproductive ecology of the big-handed crab Heterozius rotundifrons was studied at Kaikoura between November 1997 and December 1998. H. rotundifrons was found at mean densities of 7.6 per m² (± 1.4) within the middle and low shore levels and varied little between seasons. The variance! mean ratio indicated that males and females aggregated within these shore levels. The sex ratio was significantly female biased during the majority of the year. Allometric growth rates indicated that males and females reached sexual maturity at 11 mm carapace width (CW). In males, spermatozoa production occurred between 9-9.99 mm CW. Ovigerous females were present every month except February. The first broods of the year were produced in March which coincides with a decrease in the female gonado-somatic index (GSI). These broods were incubated for approximately nine months whereas broods produced in August were incubated for only five months. Female brood production appeared to be cyclical, alternating between a winter incubation period and a summer incubation period. The completed cycle takes approximately three years with two broods produced during the cycle. Fecundity increased with female size but egg mortality was quite high (19%). Instantaneous mortality rate increased with increasing brood development. Females mate when recently moulted (soft-shelled). Although females moulted through out the year, in small numbers, there was a peak in female moulting during October and November. The operational sex ratio (OSR) was male biased during all months of the year. Females released an attractant prior to moulting which initiated pre-copulatory mate guarding by the male. Once the female moulted, copulation occurred approximately 6 h later. Copulation lasted for approximately 3 h and was followed by a period of post-copulatory mate guarding. Males increased the duration of post-copulatory mate guarding if another male was present. Large males out competed small males for receptive females. Males used their large cheliped to subdue competitors and to provide protection for the soft female. Mate guarding was shown to reduce cannibalism from other females. Spermatozoa are packaged in spermatophores within the vas deferens of males but are quickly dehisced within the spermathecae (ventral-type) of newly mated females. Last male to copulate probably achieves the highest level of paternity. Postcopulatory mate guarding by the male was found and ejaculates were found in discrete packets within the spermatheca. Sperm competition appeared to be important because large males displaced small males during copulation, males left when a female was still receptive and the females could retain sperm between moults. It is concluded that H. rotundifrons probably has a polygynous mating system in which males compete for soft females (female centered competition)